Select the Memory Card or USB Stick as your preferred storage device.

Load up Command and Conquer 3: Tiberium Wars. Leave the Memory Card or USB stick in the console and turn it back on. Make sure you are not connected to the internet by turning the router off or by removing the ethernet cable. In the new Slim you will need to slide open the bottom of the console (that's the base of the console when its standing on end) and then pull firmly on the tag to remove the hard drive. On the old console you just press in the catch and lift it out. Turn off the Xbox and remove the hard drive. Transfer only your gamer profile and nothing else onto the Memory Card or USB stick. If you don't know you'll soon find out because if it isn't the right size the option to configure it with the Xbox will be greyed out. The first thing you need is an Xbox 360 Memory Card or USB stick. Credit goes to Pyro Jigamafre and Brandon1350 without whom I would never have got the achievement. If you missed this, there is a way to unlock it again but it takes a lot of effort.

Command and conquer 3 pc manual